The invention relates to a method of producing cross-sectional images using an ultrasonic imaging unit operating on the pulse-echo principle and comprising a transducer system which includes a stationary elongated array of adjacent transducer elements and having transverse electrode segments having on at least one side transverse electrode segments adjacent one another. In the method, successively and cyclically selected groups of adjacent transducer elements of the transducer system are used to produce an ultrasonic beam in response to pulsed electrical transmitter signals applied to the electrode segments. The transducer elements are also used to transmit the ultrasonic beam into a heterogeneous body, receive echoes reflected from a discontinuity in the body, and generate an electric echo signal in response to the received echoes. The invention also relates to an ultrasonic imaging unit for performing the method.
In order to produce ultrasonic images (more particularly for producing cross-sectional images) it is conventional for an ultrasonic transducer to be mechanically moved. This has various disadvantages. If the transducer is moved by hand, the scanning process is lengthy and dependent on the skill of the operator. If the transducer is moved by a motor, a relatively heavy water bath is usually required. In addition, the extra distance travelled through the water bath results in a reduction in the maximum possible image frequency.
In order to obviate these disadvantages, therefore, ultrasonic imaging units with electronic scanning have been developed, the ultrasonic beam being linearly shifted in time.
In a known ultrasonic imaging unit of the aforementioned kind (U.S. Pat. No. 3,881,466), the transducer system produces an unfocussed ultrasonic beam and the transverse resolution is determined by the width of the transducer elements. In the known device, there is a limit to which the transverse resolution can be improved by reducing the width of the transducer elements, the limit being set by the minimum width of the ultrasonic beam. Although the cross-sectional images produced by the known device are relatively distinct, it has been found in practice that still higher transverse resolution is desirable for many applications.